1. Field of the Invention
The present invention relates to pulverulent, water-dispersible, blocked polyisocyanate adducts of high storage stability which by dispersion in water and by combination with further binder dispersions are able to function as curing components in aqueous baking enamels.
2. Discussion of the Background
Aqueous dispersions of fully or partly blocked polyisocyanates have for years increasingly been used as curing agents in aqueous baking enamels. As aqueous dispersions of copolymers on an acrylic basis, various isocyanate-containing addition polymers are principally described (U.S. Pat. No. 5,314,964, DE 44 33 874). The solid copolymers have been obtainable by drying methods from the isocyanate-functionalized polymer dispersions obtained by free-radical polymerization. In some cases, with the assistance of protective colloids during drying (DE 197 11 741) and in combination with other binders, the redispersible powders described are employed in coating compositions and adhesives. Disadvantages of the known preparation methods are the need to remove fairly large amounts of water, which acts as a solvent during the free-radical polymerization, and the low blocked isocyanate functionalities content. Simple blocked polyisocyanate curing agents, present in powder form and able by dispersion to function as curing agent in aqueous baking enamels, have not been disclosed to date.
Processes for preparing high-solids aqueous dispersions from (blocked) polyisocyanates in accordance with the prior art make use of emulsifiers and protective colloids to hydrophilicize the otherwise hydrophobic polyisocyanates. These can either be mixed in physically or be incorporated into the resin structure. A disadvantage of the first method is the permanent hydrophilicity of the external emulsifiers, which leads to swelling and softening in the resultant coatings under the influence of moisture thereby ruling out exterior applications of such a dispersion. The second method is based on the reaction of (possibly partly blocked) polyisocyanates with a compound which carries a potentially hydrophilic group, after which conversion into a purely aqueous system takes place by dispersion and distillation of any cosolvent present.
For the preparation of the hydrophilicized polyisocyanate adducts, it is usually necessary, due to viscosity, to use organic auxiliary solvents (for example, EP 0 839 847 and references cited therein). The solvents, however, have to be removed with some effort from the resulting aqueous dispersions. The resulting dispersions can be designated as virtually free from auxiliary solvents. In all of the systems thus prepared, however, the limited storage stability of the dispersions is considered to be problematic.